1. Field of the Invention
This invention relates to a method for operating a light-radiant furnace which uses incandescent lamps as its light-radiant source.
2. Description of the Prior Art
Among a variety of apparatus adapted to carry out heat treatments therein, light-radiant furnaces in which light radiated from an incandescent lamp or lamps is irradiated onto objects or materials to be treated (hereinafter referred to merely as "objects") for their heat treatment have the following merits:
(1) Owing to an extremely small heat capacity of an incandescent lamp per se, it is possible to raise or lower the heating temperature promptly;
(2) The heating temperature can be easily controlled by controlling the electric power to be fed to the incandescent lamp;
(3) Since they feature indirect heating by virtue of light radiated from their incandescent lamps which are not brought into contact with the objects, there is little danger of contaminating objects under heat treatment;
(4) They enjoy less energy consumption because full-radiation-state operations of the lamps are feasible in short time periods after turning the lamps on and the energy efficiencies of the lamps are high; and
(5) They are relatively small in size and inexpensive compared with conventional resistive furnaces or high-frequency heating furnaces.
Such light-radiant heating furnaces have been used for the heat treatment and drying of steel materials and the like and the molding of plastics as well as in thermal characteristics testing apparatus and the like. Use of light-radiant furnaces have, particularly recently, been contemplated to replace the conventionally-employed resistive furnaces and high-frequency heating furnaces for carrying out certain semiconductor fabrication processes which require heating, for example, diffusion processes of dopant atoms, chemical vapor deposition processes, healing processes for crystal defects in ion-implanted layers, thermal treatment processes for electrical activation, and thermal processes for nitrifying or oxidizing the surfaces of silicon wafers. As reasons for the above move, may be mentioned the incapability of conventional heating furnaces for use in the uniform heating of larger-sized objects, thereby failing to meet the recent trend toward larger semiconductor wafer size. In addition to such advantages of light-radiant furnaces that objects under heat treatment are free from contamination, their electric properties are not deleteriously affected and the light-radiant heating furnaces require less power consumption.
In such a light-radiant furnace, there is provided a suitable conveyor system for transporting objects. By the conveyor system, each object is transported into the light-radiant furnace and then subjected to a heat treatment therein. Namely, each object is held on a carrier of the conveyor system at the loading station of objects. Thereafter, the carrier is caused to move in the light-radiant furnace in which the object held on the carrier is exposed to light radiated from the incandescent lamps so as to carry out the heat treatment. Then, the carrier is again caused to move to the unloading station of treated objects, which unloading station is located outside the light-radiant furnace. At the unloading station, the thus-treated object is unloaded from the carrier, thereby completing a single cycle of the heat treatment process. The carrier is thereafter moved again to the loading station and loaded with the next object to be treated. Then, the heat treatment process of the next object is carried out in the same manner.
Heat treatment of objects are generally carried out in such a manner as mentioned above. Depending on the kinds or types of objects to be treated, their heating temperatures and heating time periods must be strictly controlled. Accordingly, objects of the same kind or type must be heat-treated under the exactly same conditions.
To achieve such a heat treatment, it may for example be contemplated to keep incandescent lamps used as the light-radiant source such as halogen incandescent lamps on lighting with a constant power so as to make the radiated light energy constant in the light-radiant furnace and, while maintaining the above state, to hold each object for a predetermined constant period of time in the light-radiant furnace for its heat treatment. This method is however accompanied by a drawback that the electric power is significantly wasted, because the incandescent lamps are continuously lit with the same electric power as that supplied while effecting the heat treatment of an object even when no object is present in the light-radiant furnace, in other words, during the waiting period.
It may also be contemplated, for saving the power consumption, to light the incandescent lamps with a constant power and for a predetermined time period after transportation of each object into the light-radiant furnace so as to carry out the heat treatment of the object for the predetermined time period and to keep the incandescent lamps off during any periods other than heat treatment periods. This method is however accompanied by such shortcomings as will be mentioned below.
In an incandescent lamp, the filament coil contains some parts where the diameter of the filament is smaller than the remaining parts thereof due to non-uniformity in diameter of the starting filament. Furthermore, the filament of the filament coil becomes thinner at certain parts thereof in the course of its use due to uneven vaporization of the metal making up the filament coil, which uneven vaporization is caused by irregularity of the coil pitch of the filament coil, local non-uniformity of the halogen cycle if the incandescent lamp is a halogen incandescent lamp, or other causes. When using incandescent lamps by repeatedly turning them on and off many times in order for example to subject a number of objects to heat treatment successively, there is a big difference between the resistance of the filament coil of each incandescent lamp prior to turning it on and that during the full-radiation-state operation thereof. Thus, a rush current as large as several times (about 7-12 times) the current supplied during the full-radiation-state operation is caused to flow through the filament coil at every time of turning on, thereby making the filament coil hotter at smaller-diametered parts than the remaining parts thereof and accelerating vaporization of the metal at the smaller-diametered parts. Accordingly, the filament coil is eventually burnt out at one of the smaller-diametered parts, thereby shortening service life of the lamp. This means that such incandescent lamps have to be replaced by fresh incandescent lamps rather often, resulting in a high running cost of the light-radiant furnace from the long-term viewpoint.